@Nicole2592Climate change is a bear. We live at 8,500' elevation and no houses up here have a/c. Summers are getting much hotter (compared to what most up here are use to) and sometimes our cabin get's in the 90's. What would make your temps hard for me is the humidity on top of 100 degree heat. Something unheard of 15 years ago. We upgraded our ceiling fans to have heavyduty motors that really move the air, and also have portable floor fans in all three bedrooms. As I live up here, I can say the floor fans really help my comfort level since I can aim them right at me where ever i move.
Our listing clearly specifies we are in the mountains and no one has a/c. Our guests from the southern states don't'seem to mind the warmer summer days, as most are outside recreating anyway. And those same southerners say our evening in August dictate to them to put sweaters on. I would emphasize the age of your rental, lack of insulation comes with the charm. Let them know you are already offering space heaters which is very considerate. Offering floor fans might be something to consider, (to get a direct breeze) but that's the most I would do. I have no plans to put in A/c.